Mechanochemical Synthesis of a Sodium Anion Complex [Na+(2,2,2-cryptand)Na–] and Studies of Its Reactivity: Two-Electron and One-Electron Reductions

Group 1 metal molecular chemistry is dominated by a +1 oxidation state, while a 0 oxidation state is widespread in the metals. A more exotic, yet still available, oxidation state of group 1 metal is −1, i.e., alkalide. Reported as early as the 1970s, the alkalides appear in every modern inorganic chemistry textbook as an iconic chemical curiosity, yet their reactivity remains unexplored. This is due to their synthetic hurdles. In this work, we report the first facile synthesis of the archetypical alkalide complex, [Na+(2,2,2-cryptand)Na–], which allows us to unveil a versatile reactivity profile of this once exotic species.

All structures were solved using XT [3] and refined by XL [4] using the Olex2 interface [5].All non-hydrogen atoms were refined anisotropically and hydrogen atoms were positioned with idealised geometry.The displacement parameters of the hydrogen atoms were constrained using a riding model with U (H) set to be an appropriate multiple of the U eq value of the parent atom.

Section 2. Computational details and data
2.1 General Procedure: Geometry optimisation and vibrational frequency calculations were completed for each structure considered in this work; inputs were created using experimental SCXRD structures where appropriate.The calculations utilised the hybrid DFT functional B3LYP along with a 6-311+g** basis set for all atoms [7] [8], in the case of molecules that possessed spin unrestricted B3LYP was used.The effects of THF molecules as a solvent were included through the implementation of the integral equation formulation of the polarizable continuum model (IEFPCM) and Grimme's D3 dispersion correction with Becke-Johnson damping was used as implemented in Gaussian 16 [9].Each structure optimised in this work was confirmed to be a minimum on the potential energy surface, through observation of 3N-6 real vibrational frequencies.The geometry optimisation, vibrational frequency, natural bond orbital charge analysis and NMR shielding tensor calculations utilised in this work were all completed using the Gaussian 16 Rev.C.01 package.Molecular orbital diagrams and spin density orbital diagrams were generated using ChemCraft 1.8 based on the relevant optimised geometry from the Gaussian 16 calculation [9] [10] [11].

Gibbs free energy of formation calculations:
In order to assess the thermodynamic feasibility of the 1-electron and 2-electron reaction routes for the reaction between 1 and tritylamine the Gibbs free energy of formation for each process was investigated.
The electronic energy (EE), thermal correction to Gibbs free energy (ΔG corr ) and thermally corrected Gibbs free energy (ΔG (298) ) for each species were obtained from the calculations and are shown in Table S3 alongside the sum of the ΔG (298) for the reactants and both sets of products.The Gibbs free energy of formation (ΔG f ) for the two potential reaction routes was calculated using Equation S1, yielding values of 196 and -226 kJmol -1 for the 1 and 2-electron reaction routes respectively (rounded to the nearest whole number).
A similar approach was attempted for the reaction of 1 with azobenzene, however, the hypothesised product of the 2-electron reaction (uranium complex analogue) did not appear to resemble a minimum energy structure.Each attempt to optimise the hypothesised product resulted in breakage of the 4-membered ring structure in the centre of the complex, suggesting that the product itself is not feasible.

2. 3 4
Molecular orbital diagrams of 1, 2, The following molecular orbital diagrams are based on the optimised minimum structures of 1, 2 and 4 are show in Figures S11-13 respectively.

Figure S26 :
Figure S26: Diagrams of the HOMO (left) and LUMO (right) of the optimised structure of 1 generated using Chemcraft 1.8 with a contour value of 0.02.

Figure S27 :
Figure S27: Diagrams of the HOMO (left) and LUMO (right) of the optimised structure of 2 generated using Chemcraft 1.8 with a contour value of 0.02.

Figure S28 : 4 Figure S29 :
Figure S28: Diagrams of the α highest energy SOMO (left -with both side and top view) and LUMO (right -with both a side and top view) of 4 generated using Chemcraft 1.8 with a contour value of 0.02.2.4 Spin density analysis of 4

Table S6 .
The EE, ΔG corr , ΔG (298) and the sum of ΔG (298) for the reactants and both sets of products for the reaction between 1 and tritylamine.